How do you end the call by using the sleep/wake button on an iphone instead of turning off the screen?

When I am on a call with my iPhone, and I want to end the call, I press the sleep/wake button to end it. Usually this works to end the call, But on multiple occurrences, it has just locked the phone and basically just turned off the screen without ending the call. How do you differentiate ending the call and turning off the screen/locking it when using the sleep/wake button?